The processing steps for fabricating steel products generate various kinds of waste waters that are discharged. Hazardous substances may be contained in those waste waters if they remain untreated. Waste water containing hazardous substances are sent from various processing steps to a waste water treatment process. In a waste water treatment plant, the waste water is neutralized, sedimented, and otherwise treated, and hazardous components are treated as refractory substances and separated by sedimentation (see JP-A-7-268659 and JP-A-2002-200494).
For example, a surface treatment or activating process is performed as a post treatment of a galvanizing process in a hot-dip galvanizing line. In the surface treatment, galvanized steel sheets are rinsed with a surface treatment liquid containing P after a cleaning process (see, for example, JP-A-2007-92093). Since the surface treatment liquid and the waste water from the rinsing water contain a P component of the surface treatment or activating process, it is impossible to drain out them directly. Therefore, waste water from a surface treatment or activating process is drained to a drainage pit within the galvanizing line. The waste water is sent from the drainage pit to a waste water treatment plant, where the waste water is neutralized, sedimented and otherwise treated. The waste water is sedimented and separated while treating the P component as a refractory substance.
However, if the P concentration of the waste water sent to the waste water treatment plant is high, it is impossible to neutralize, sediment and otherwise treat the waste water in the waste water treatment plant. Consequently, an upper limit of the P concentration of the waste water that can be sent into the waste water treatment plant is established. It is necessary to immediately stop sending of the waste water into the waste water treatment plant if the P concentration of the waste water from the drainage pit exceeds the upper limit of P concentration. If the flow of waste water into the waste water treatment plant is stopped, operation of the galvanizing line is typically stopped.
It is therefore customary to employ a waste water buffer tank capable of storing the waste water to prevent operation of the hot-dip galvanizing line from being stopped when waste water exceeding the upper limit of P concentration is produced. Feeding the waste water into the waste water treatment plant is stopped when the P concentration of the waste water exceeds the upper limit of P concentration. The waste water is then stored in the waste water buffer tank.
However, if the operation is run after stopping the flow of waste water into the waste water treatment plant, the operation can run only for a short time until the waste water buffer tank becomes full.
It is also possible to dilute the waste water by adding diluting water into a waste water buffer tank such that the P concentration of the waste water becomes lower than the upper limit of P concentration such that the waste water can be sent to a waste water treatment plant. The thus diluted waste water can be sent to the waste water treatment plant. However, this method has the problem that the amount of waste water increases.
It could therefore be helpful to provide a method for controlling the concentration of P in cleaning solution waste water in such a way that sending waste water with high P concentrations and large amounts of waste water containing P into a waste water treatment plant can be prevented. This can assist in avoiding interrupting the operation of the galvanizing line.